Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas is essentially a gaseous mixture of stable molecules that contain the elements carbon (C), hydrogen (H), and oxygen (O), arguably the three most-important elements for sustaining life. Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses, as is well-known. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power.
Syngas can be produced, in principle, from virtually any material containing C, H, and O. Such materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials. It is preferable, however, to utilize a renewable resource to produce syngas because of the rising economic, environmental, and social costs associated with fossil resources.
There exist a variety of conversion technologies to turn carbon-containing feedstocks into syngas. Typical approaches utilize a combination of one or more steps comprising gasification, pyrolysis, steam reforming, and partial oxidation. As is recognized in the art, various advantages can be realized depending on the specific technology and process configuration selected.
In the chemical-process industries, including biomass refining, it is widely held that economies of scale are realized as processes are scaled up. While certain unit operations, such as filtration, tend to scale linearly, many common units such as reactors, holding tanks, and distillation columns tend to scale with a scale-up exponent n less than unity. Typically n=0.6−0.8, meaning that a doubling of capacity equates to something less than a doubling of capital cost. For example, when the scaling exponent n=0.6, the expected capital cost for a doubling of capacity is just 50% more than the base case (20.6=1.5). One reason for such economy of scale is that capacity tends to increase with volume, but costs for materials of construction tend to increase with surface area. Mathematically, this phenomenon predicts that the ratio of materials costs would increase to the two-thirds power with the ratio of volumes; indeed, n=2/3 is fairly typical.
For capital-intensive unit operations, there is thus economic pressure to make units as large as technically and practically possible. There are, however, several drawbacks to constructing massive-scale plants, especially for processing biomass.
First, there can be engineering uncertainties associated with scale-up. Even when a pilot plant is constructed and operated, the jump to the manufacturing scale is typically several orders of magnitude. Fundamental factors such as heat transfer, mass transfer, and the like can change in unpredictable ways, causing undesirable performance and sometimes necessitating design changes.
Second, as biomass-refining plants increase in size, a point is reached wherein the cost of transporting large quantities of biomass can cause the operating costs to be excessive. This is especially true for ultra-low-density biomass, such as straws and grasses, but is also true for low-density wood feedstocks. Typical approaches in the industry balance the cost of biomass transportation with economies of scale, implying a necessary trade-off.
Third, large-scale installations invariably require primarily engineered systems that cannot benefit from the standardization and utilization of mass-production techniques that can be applied to modular designs.
Modularization may be viewed as the grouping together of a set of units, or modules, to achieve a specific function (Schug and Realff, 1996). The concept of modularity has shown development in a range of areas, including pump design (Chynoweth, 1987); space modules (Cooper, 1990); oil drilling platforms (North, 1995); and manufactured parts (Sorokin, 1989). The purposes of development in these areas include faster lead times and flexibility of operation. Also, modularization may reduce the total amount of engineering development time and expense for each facility and for subsequent designs. Other noted benefits are an increase in the number of designs through combinations of modules and rapid deconstruction and reconstruction of modular systems (Schug and Realff, 1996).
A significant portion of biomass feedstock costs—especially from forests—can be attributed to the handling associated with moving them from their point of production to their end point of conversion or end-use (Sokhansanj, 2002). Traditionally, handling includes harvesting, chipping, loading onto trucks, and transportation to the end-use point. Handling solid forms of biomass is expensive for a number of reasons, including the number of operations required and the low bulk density of the feedstocks (Badger, 2002), which cause high transportation costs.
There exists a desire to overcome the large expenses associated with biomass handling. One way to minimize handling expenses is to reduce and/or optimize average transport distances for feedstock to conversion units. Therefore, methods and systems are necessary to determine an optimal number and distribution of modular units spatially located within a region of land.
In view of the aforementioned needs in the art with respect to modularity and distribution, what are especially needed are biomass-to-syngas methods and systems that are flexible, efficient, scalable, and ultimately cost-effective at virtually any scale of operation.